Perpendicular magnetic recording more advantageous for high-density recording in principle than longitudinal magnetic recording is increasing the recording density of a hard disk drive (HDD) by about 40% per year. Even when using this perpendicular magnetic recording method, however, it is probably not easy to increase the recording density because the problem of thermal decay becomes conspicuous.
“A high-frequency magnetic field assisted recording method” has been proposed as a recording method capable of solving this problem. In this high-frequency magnetic field assisted recording method, a high-frequency magnetic field near the resonance frequency of a magnetic recording medium, which is much higher than a recording signal frequency, is locally applied to the medium. Consequently, the medium resonates, and the coercive force (Hc) in that portion of the medium to which the high-frequency magnetic field is applied becomes half or less the original coercive force. By superposing a high-frequency magnetic field on a recording magnetic field by using this effect, magnetic recording can be performed on a medium having a higher coercive force (Hc) and higher magnetic anisotropic energy (Ku). If a high-frequency magnetic field is generated by a coil, however, it is difficult to efficiently apply the high-frequency magnetic field to a medium.
As high-frequency magnetic field generating means, therefore, methods using spin torque oscillators have been proposed. In techniques disclosed in these methods, a spin torque oscillator includes a spin transfer layer, an interlayer, a magnetic material layer (oscillation layer), and electrodes. When a direct current is supplied to the spin torque oscillator through the electrodes, a spin torque generated by the spin transfer layer causes the ferromagnetic resonance of magnetization of the magnetic material layer. As a consequence, the spin torque oscillator generates a high-frequency magnetic field. Since the size of the spin torque oscillator is about a few ten nm, the generated high-frequency magnetic field locally exists in a region of about a few ten nm in the vicinity of the spin torque oscillator. Furthermore, the longitudinal component of the high-frequency magnetic field can efficiently resonate a perpendicularly magnetized medium, and this makes it possible to largely decrease the coercive force of the medium. As a result, high-density magnetic recording is performed in only a portion where a recording magnetic field generated by a main magnetic pole and the high-frequency magnetic field generated by the spin torque oscillator are superposed on each other, so a medium having a high coercive force (Hc) and high magnetic anisotropic energy (Ku) can be used. Accordingly, the problem of thermal decay during high-density recording can be avoided.
To implement a high-frequency magnetic field assisted recording head, it is important to design and manufacture a spin torque oscillator capable of stably oscillating with a low driving current, and generating a longitudinal high-frequency magnetic field that sufficiently resonates medium magnetization.
A maximum current density that can be supplied to the spin torque oscillator is 2×108 A/cm2 when the element size is, e.g., about 70 nm. If the current density is higher than that, the characteristics deteriorate due to, e.g., heat generation and migration of the spin torque oscillator. This makes it important to design a spin torque oscillator capable of oscillating at as low a current density as possible.
On the other hand, to sufficiently resonate medium magnetization, the intensity of the longitudinal high-frequency magnetic field is reportedly desirably 10% or more of the anisotropic magnetic field (Hk) of the medium. Examples of a means for increasing the intensity of the longitudinal high-frequency magnetic field are increasing the saturation magnetization of the oscillation layer, increasing the thickness of the oscillation layer, and increasing the rotational angle of magnetization of the oscillation layer. Unfortunately, all these means increase the driving current.
As described above, decreasing the density of the driving current is inconsistent with increasing the intensity of the longitudinal high-frequency magnetic field, and it is desirable to implement a spin torque oscillator capable of achieving both of these demands.